1. Field of the Invention
The invention refers to a mixture for the manufacture of hydraulic setting fiber-containing shaped elements, especially according to the wet process, containing a calcareous respectively lime-releasing binder, fibers and/or fiber-type products--in particular cellulosic fibers--and highly reactive pozzuolanic silicic acid as well as, if required, usual fillers and/or additives, especially mixing auxiliaries, agents for improvement of rheology and/or structure.
2. Description of the Prior Art
In accordance with the well-known wet process such mixtures are dewatered on Hatschek equipment and moulded. It is generally known that the subsequent setting of cement can take place at room temperature or at increased temperatures, e.g., at 40.degree. to 100.degree. C., in which case the relative humidity of the air should exceed 85%. The hardening process following the setting of the hydraulic binder may take place at room temperature or under the hydrothermal conditions in the autoclave.
The powdered silicic acid employed in this process is a pozzuolanic product of especially fine particles obtained in large qunantities in silicon (Si) electro-metallurgy for example. In international usage this product is known as "condensed silica fume (CSF)" while in German the term "kondensierter Kieselsaurestaub or Kieselsaurerauch" is employed. Another similar material is diatomite.
Up until now the CSF has been employed in such mixtures in an amount of from 5 to 40% by weight in relation to the dry mixture. However, shaped elements made from such mixtures, while showing the desired strength properties, become brittle and tend to form cracks event at minimum exposure to effort, e.g. may occur with changes of humidity during drying prior to application of varnish to the surface. A further drawback is the reduction in productivity when working in accordance with the wet process because CSF exercises a negative influence on the dewatering behaviour of the mixture.
In the course of research work it was discovered that this tendency to embrittlement combined with formation of cracks in finished shaped products could be avoided by addition of highly reactive pozzuolanic silicic acid (CSF) a quantity of less than 5% by weight in relation to the dry mixture. On the other hand, unfortunately, with a mixture of this type it is, as a result, not possible to obtain sufficiently satisfactory strength values, so that this positive result in general does not offer a solution to the problem.